HISTOLOGY OF EPITHELIAL AND CONNECTIVE TISSUES. 33 



epidermis enables it to resist, within limits, the injurious influences of 

 acids, alkalies, and poisons. 



2. To promote absorption. Inasmuch as the skin and mucous membranes 

 cover the surfaces of the body, it is obvious that all nutritive material 

 entering the body must first traverse the epithelial tissue. Owing to 

 their density, however, the epithelial cells covering the skin play but a 

 feeble role as absorbing agents in man and the higher animals. The 

 epithelium of the mucous membrane of the alimentary canal, particularly 

 that of the small intestine, is especially adapted, from its situation, con- 

 sistency, and properties, to play the chief role in the absorption of new 

 materials into the blood. The epithelium lining the air-vesicles of the 

 lungs is engaged in promoting the absorption of oxygen and the exhalation 

 of carbon dioxid. 



3. To form secretions and excretions. Each secretory gland connected with 

 the surfaces of the body is lined by epithelial cells, which are actively con- 

 cerned in the formation of the secretion peculiar to the gland. Each 

 excretory organ is similarly provided with epithelial cells, which are en- 

 gaged either in the production of the constituents of the excretion or in 

 their removal from the blood. 



2. THE CONNECTIVE TISSUES. 



The connective tissues, in their collective capacity, constitute a frame- 

 work which pervades the body in all directions, and, as the name implies, 

 serve as a bond of connection between the individual parts, at the same time 

 affording a basis of support for the muscle, nerve, and gland tissues. The 

 connective-tissue group includes a number of varieties, among which may 

 be mentioned the areolar, adipose, retiform, white fibrous, yellow elastic, 

 cartilaginous and osseous. Notwithstanding their apparent diversity, they 

 possess many points of similarity. They have a common origin, developing 

 from the same embryonic material; they have much the same structure, 

 passing imperceptibly into one another, and perform practically the same 

 functions. 



Areolar Tissue. This variety is found widely distributed throughout 

 the body. It serves to unite the skin and mucous membrane to the struc- 

 tures on which they rest; to form sheaths for the support of blood -vessels, 

 nerves, and lymphatics; to unite into compact masses the muscular tissue of 

 the body, etc. Examined with the naked eye, it presents the appearance 

 of being composed of bundles of fine fibers interlacing in every direction. 



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